1. Field of the Invention
This invention relates to techniques for nondestructive testing of conductors for constrictions and discontinuities that escape standard conductivity testing.
2. Description of the Prior Art
A significant and increasingly difficult problem with packaging and interconnection systems in complex circuits, such as circuits in computers with high levels of integration, is the detection of incipient opens that escape standard conductivity testing of conductors.
A number of third harmonic techniques have been used to study electrical conductors. Typical of such third harmonic testing methods is that described in U.S. Pat. No. 3,733,545, Elsner et al., Method for Locating Nonlinear Mechanical Junctions of Metallic Electrical Conductors, May 15, 1973. Another third harmonic measuring technique is described in U.S. Pat. No. 3,500,188, Whitley et al., Method and Means for Measuring Constriction Resistance Based on Nonlinearity, Mar. 10, 1970.
Fault location techniques involving the second harmonic have also been used. For example, U.S. Pat. No. 3,299,351, D. L. Williams, Apparatus for Detecting Faults in Buried Cables Including Means for Applying a Composite Signal Having Fundamental and Even Harmonic Frequency Components, Jan. 17, 1967, describes a technique for providing a composite signal of a fundamental frequency and even harmonics. Williams deliberately impresses the second harmonic as the test current--measures the polarity of the potential due to the composite signal flowing into the ground at the fault location--to find the location.
Prior art testers have been minimally successful in locating current constricting defects and other nonlinearities which accompany incipient faults, because the electrical effects of linear resistances normally present along the conductor swamp the electrical effects of the nonlinearity being tested for. Attempts to overcome these "good line" signals can result in destruction of good lines, aggravation of incipient faults without detection of the fault, or false detection of faults in good circuits.
Testers in the prior art have not operated by impressing a DC biased fundamental frequency AC signal on the conductor to be tested (so as to cause both linear characteristic "good line" signals including second harmonics and nonlinearity signals including second harmonics), balancing out the second harmonic signals from linear impedance characteristics of the "good line" conductor, and detecting the second harmonic signals caused by the fault.